PVD is a technology by which thin metallic and/or ceramic layers can be sputter-deposited onto a substrate. Sputtered materials come from a target, which serves generally as a cathode in a standard radio-frequency (RF) and/or direct current (DC) sputtering apparatus. For example, PVD is widely used in the semiconductor industry to produce integrated circuits.
A relatively new application for sputtering technologies is fabrication of FPDs, such as, for example, LCDs. The LCD market has experienced rapid growth. This trend may accelerate in the next few years due to the diversified applications of LCDs in, for example the markets of laptop personal computers (PCs), PC monitors, mobile devices, cellular phones and LCD televisions.
Aluminum can be a particularly useful metal in forming LCDs, and it accordingly can be desired to form aluminum-comprising physical vapor deposition targets. The targets can contain a small content (less than or equal to about 100 parts per million (ppm)) of doping elements. The aluminum, with or without small additions of dopants, is generally desired to be deposited to form a layer of about 300 nm which constitutes the reflecting electrode of LCD devices. Several factors are important in sputter deposition of a uniform layer of aluminum having desired properties for LCD devices. Such factors including: sputtering rate; thin film uniformity; and microstructure. Improvements are desired in the metallurgy of LCD aluminum targets to improve the above-discussed factors.
LCD targets are quite large in size, a typical size being 860×910×19 mm3, and are expected to become bigger in the future. Such massive dimensions present challenges to the development of tooling and processing for fabrication of suitable aluminum-comprising targets.
Various works demonstrate that three fundamental factors of a target can influence sputtering performance. The first factor is the grain size of the material, i.e. the smallest constitutive part of a polycrystalline metal possessing a continuous crystal lattice. Grain size ranges are usually from several millimeters to a few tenths of microns; depending on metal nature, composition, and processing history. It is believed that finer and more homogeneous grain sizes improve thin film uniformity, sputtering yield and deposition rate, while reducing arcing. The second factor is target texture. The continuous crystal lattice of each grain is oriented in a specific way relative to the plane of target surface. The sum of all the particular grain orientations defines the overall target orientation. When no particular target orientation dominates, the texture is considered to be a random structure. Like grain size, crystallographic texture can strongly depend on the preliminary thermomechanical treatment, as well as on the nature and composition of a given metal. Crystallographic textures can influence thin film uniformity and sputtering rate. The third factor is the size and distribution of structural components, such as second phase precipitates and particles, and casting defects (such as, for example, voids or pores). These structural components are usually not desired and can be sources for arcing as well as contamination of thin films.
In order to improve the manufacture of LCD targets it would be desirable to accomplish one or more of the following relative to aluminum-based target materials: (1) to achieve predominate and uniform grain sizes within the target materials of less than 100 μm; (2) to have the target materials consist of (or consist essentially of) high purity aluminum (i.e. aluminum of at least 99.99% (4N) purity, and preferably at least 99.999% (5N) purity, with the percentages being atomic percentages); (3) to keep oxygen content within the target materials low; and (4) to achieve large target sizes utilizing the target materials.
The thermomechanical processes (TMP) used traditionally to fabricate LCD targets can generally only achieve grain sizes larger than 200 μm for 5N Al with or without dopants. Such TMP processes involve the different steps of casting, heat treatment, forming by rolling or forging, annealing and final fabrication of the LCD target. Because forging and rolling operations change the shape of billets by reducing their thickness, practically attainable strains in today's TMP processes are restricted. Further, rolling and forging operations generally produce non-uniform straining throughout a billet.
The optimal method for refining the structure of high purity aluminum alloys (such as, for example, 99.9995% aluminum) would be intensive plastic deformation sufficient to initiate and complete self-recrystallization at room temperature immediately after cold working.
High purity aluminum is typically provided as a cast ingot with coarse dendrite structures (FIG. 1 illustrates a typical structure of as-cast 99.9995% aluminum). Forging and/or rolling operations are utilized to deform the cast ingots into target blanks. Flat panel display target blanks are optimally to be in the form of large thin plates. The total strains which can be obtained for any combination of forging and/or rolling operations can be expressed as ε=(1−h/H0)*100%; where H0 is an ingot length, and h is a target blank thickness. Calculations show that possible thickness reductions for conventional processes range from about 85% to about 92%, depending on target blank size to thickness ratio. The thickness reduction defines the strain induced in a material. Higher thickness reductions indicate more strain, and accordingly can indicate smaller grain sizes. The conventional reductions of 85% to 92% can provide static recrystallization of high purity aluminum (for instance, aluminum having a purity of 99.9995% or greater) but they are not sufficient to develop the fine and uniform grain structure desired for flat panel display target materials. For example, an average grain size after 95% rolling reduction is about 150 microns (such is shown in FIG. 2). Such grain size is larger than that which would optimally be desired for a flat panel display. Further, the structures achieved by conventional processes are not stable. Specifically, if the structures are heated to a temperature of 150° C. or greater (which is a typical temperature for sputtering operations), the average grain size of the structures can grow to 280 microns or more (see FIG. 3). Such behavior occurs even after intensive forging or rolling.
FIG. 4 summarizes results obtained for a prior art high purity aluminum material. Specifically, FIG. 4 shows a curve 10 comprising a relationship between a percentage of rolling reduction and grain size (in microns). A solid part of curve 10 shows an effect of rolling reduction on a 99.9995% aluminum material which is self-recrystallized at room temperature. As can be seen, even a high rolling reduction of 95% results in an average grain size of about 160 microns (point 12), which is a relatively coarse and non-uniform structure. Annealing at 150° C. for 1 hour significantly increases the grain size to 270 microns (point 14). An increase of reduction to 99% can reduce the grain size to 110 microns (point 16 of FIG. 4), but heating to 150° C. for 1 hour increases the average grain size to 170 microns (point 18 of FIG. 4).
Attempts have been made to stabilize recrystallized high purity aluminum structures by adding low amounts of different doping elements (such as silicon, titanium and scandium) to the materials. A difficulty that occurs when the doping elements are incorporated is that full self-recrystallization can generally not be obtained for an entirety of the material, and instead partial recrystallization is observed along grain boundaries and triple joints. For example, the structure of a material comprising 99.9995% aluminum with 30 ppm Si doping is only partly recrystallized after rolling with a high reduction of 95% (see FIG. 6) in contrast to the fully recrystallized structure formed after similar rolling of a pure material (see FIG. 2). Accordingly, additional annealing of the rolled material at a temperature of 150° C. for about 1 hour is typically desired to obtain a fully recrystallized doped structure. Such results in coarse and non-uniform grains (see FIG. 7).
FIG. 5 illustrates data obtained for 99.9995% aluminum with a 30 ppm silicon dopant. The curve 20 of FIG. 5 conforms to experimental data of 99.9995% aluminum with 30 ppm silicon after rolling with different reductions. A dashed part of the curve 20 corresponds to partial self-recrystallization after rolling, while a solid part of the curve corresponds to full self-recrystallization. The full self-recrystallization is attained after intensive reductions of more than 97%, which are practically not available in commercial target fabrication processes. The point 22 shows the average grain size achieved for the as-deformed material as being about 250 microns, and the point 24 shows that the grain size reduces to about 180 microns after the material is annealed at 150° C. for 1 hour. The points 22 and 24 of FIG. 5 correspond to the structures of FIGS. 6 and 7.
For the reasons discussed above, conventional metal-treatment procedures are incapable of developing the fine grain size and stable microstructures desired in high purity aluminum target materials for utilization in flat panel display technologies. For instance, a difficulty exists in that conventional deformation techniques are not generally capable of forming thermally stable grain sizes of less than 150 microns for both doped and non-doped conditions of high purity metals. Also, particular processing environments can create further problems associated with conventional metal-treatment processes. Specifically, there is a motivation to use cold deformation as much as possible to refine structure, which can remove advantages of hot processing of cast materials for healing pores and voids, and for eliminating other casting defects. Such defects are difficult, if not impossible, to remove by cold deformation, and some of them can even be enlarged during cold deformation. Accordingly, it would be desirable to develop methodologies in which casting defects can be removed, and yet which achieve desired small grain sizes and stable microstructures.